The US 2012/230886 A1 discloses an overlay device with channels on a bottom side that can be placed upon a microscope slide carrying a biological sample. Openings at the ends of the channels allow for the inlet of reagents into said channels at one end and their outlet at the other end. A microscope slide with such an overlay device can be placed into an alignment fixture with inlet ports and outlet ports that are received by the respective openings of the overlay device.